The present invention is directed generally to toughening copolymer additives such as alkene-(meth)acrylate ester copolymer additives which when incorporated into adhesive compositions, such as cyanoacrylate adhesive compositions, increase the toughness of the adhesive upon cure.
Cyanoacrylates are highly reactive monomers that undergo rapid anionic polymerization reactions initiated by minute amounts of basic or nucleophillic species. It is generally known that small amounts of acidic or electrophilic species will retard or inhibit this reaction. As a consequence of their extreme anionic reactivity, commercial formulations of cyanoacrylate monomers usually contain small amounts of acidic stabilizers that are intended to be sufficient to ensure a reasonable shelf-life for the product, but not so excessive as to render the product inactive when it is applied to the surface of a substrate. If too little stabilizer is added, the product will be prone to premature polymerization and if too much is added it will be less active and function less effectively as an adhesive. The commercially available ethylene/methyl acrylate toughening additives, are also problematic in this respect because they often contain small amounts of materials, e.g. acids, that result in the over- or under-stabilization of the total adhesive composition.
While cyanoacrylate adhesives are useful for many applications, they inherently lack sufficient toughness and are often too brittle for certain applications. Attempts have been made to eliminate post-cure embrittlement, through the addition of various types of additives, and particularly toughening additives, which generally have elastomeric properties. For example, copolymers formed from the copolymerization of acrylate esters with olefins have been added as modifiers to cyanoacrylate adhesive compositions to impart toughening properties and lower brittleness of the cured product. In particular, ethylene-methyl acrylate copolymers are sold commercially by DuPont under the trademark Vamac and have been used as toughening additives for cyanoacrylate adhesives.
Loctite Corporation""s U.S. Pat. No. 4,440,910 discloses a cyanoacrylate adhesive composition which contains a monomeric ester of 2-cyanoacrylic acid and about 0.5% to about 20% by weight of an elastomeric polymer selected from the group consisting of elastomeric copolymers of a lower alkene monomer and (i) acrylic acid esters, (ii) methacrylic acid esters or (iii) vinyl acetate. Acrylic rubbers disclosed in this patent include ethylene-methyl acrylates under the trade name Vamac N-123 and Vamac B-124. These rubbers, as well as other Vamac products, for example, Vamac-G and Vamac-D, either contain free carboxylic acid functionalities and/or impurities which cause a slowing of the cure rate of cyanoacrylate adhesive compositions or a decrease in shelf-life when incorporated therein.
It is generally accepted in the reactive adhesive art that increased toughness and lower brittleness is achievable if toughening additives can be solvated by the uncured reactive monomer and subsequently undergo a phase separation from the adhesive matrix during the curing process.. The ability of these additives to be solvated by the cyanoacrylate monomer results in a demonstrable phase separation of the additive during polymerization of the cyanoacrylate monomer. Phase separation is generally accepted as a necessary condition for increased adhesive toughness. One problem with some commercially available ethylene-methyl acrylate copolymers is that they are only partially solvated by cyanoacrylate monomers. Thus, when cyanoacrylate adhesives which incorporate these commercially available toughening additives are cured, polymerization-induced phase separation, and hence toughening, is not optimized.
Compatibility of the toughening additive with the cyanoacrylate monomer is an important feature. For example, as noted above the toughening agent itself should not be so acidic that it significantly slows down the cure rate, and not so basic or nucleophilic that it cures prematurely. Known commercially available ethylene-acrylate copolymers are also problematic in this respect because they often contain trace amounts of carboxylic functional groups, which are known to cause a slowing of the cure rate of the adhesive In addition, these functionalities contribute to reduced activity upon storage.
Commercially available toughening copolymers used in cyanoacrylate adhesive formulations are generally prepared from olefin monomers having three carbons or less. In general, for formation of the copolymers, olefins with greater than three carbons are difficult to polymerize with alkyl acrylate esters by free radical polymerization due to their low reactivity toward free radicals and their increased tendency to undergo a chain transfer rather than a propagation reaction.
U.S. Pat. No. 3,183,217 discloses a process for copolymerizing an alkene with a (meth)acrylic acid ester. The ""217 patent discloses higher alkenes such as 1-hexene, as well as lower alkenes, as being useful materials for copolymerization with (meth)acrylic acid esters. This patent discloses admixing the alkenes with (meth)acrylic acid esters with equimolar amounts of a Lewis acid per mole of the polar vinyl monomer, i.e., (meth)acrylic acid ester, and copolymerizing the resulting admixture in the presence of a free radical initiator, under anhydrous conditions and at a temperature of about xe2x88x9278xc2x0 to about 175xc2x0 C. Using this method and the ratio of reactants disclosed therein, however, limits the amount of olefin which can be incorporated into the final copolymer product.
It is generally desired to have higher levels of olefin relative to (meth)acrylic acid ester incorporated into the copolymer product in order to avoid an unwanted plasticization of the cured adhesive. If the olefin incorporated into the copolymer product is too low, a plasticization rather than toughening of the cured adhesive occurs, which would be manifested by an incomplete or absence of phase separation of the copolymer from the adhesive matrix during cure. Moreover, the optimal amount of olefin for incorporation into the copolymer for adhesive toughening may change depending on the cyanoacrylate monomer used in the adhesive composition.
It would therefore be desirable to have a means of varying the olefin content in the olefin/(meth)acrylate ester copolymers in accordance with the cyanoacrylate monomer chosen in order to control the balance between toughening and plasticization. Thus, there is a need for a process of polymerization of an alkene (olefin) with a (meth)acrylate ester wherein the reaction conditions can be varied in order to increase the olefin content and vary the molecular weight of the resulting copolymer to achieve adhesive toughening.
It would also be beneficial to achieve toughening without the reactivity and stabilization difficulties associated with commercial toughening copolymer additives. Therefore, there is a need for curable adhesives containing toughening copolymer additives with improved solubility in cyanoacrylate monomers, as well as copolymers which do not contain trace amounts of interfering functional groups, additives, or stabilizers that can shorten the shelf life of the adhesive monomer or reduce its activity.
It would therefore be desirable to provide toughening copolymers which have been synthesized de novo from olefin and (meth)acrylate ester monomers by a method wherein the resulting copolymer products are free of interfering functional groups, additives or stabilizers. Moreover, it would be desirable to provide a method for modifying existing commercial olefin (meth)acrylate and olefin/alkenoic acid copolymers in order to eliminate acidic functional groups which are known to affect the cure rate of cyanoacrylate monomers, and by so doing make them suitable for use as toughening additives of cyanoacrylate adhesives.
The present invention provides new copolymer toughening additives which are substantially free of acidic functionality, and acidic or basic impurities and which are the reaction of an olefin monomer containing between 2 to 20 carbon atoms (olefin C2-20) and a (meth)acrylate ester. These copolymer toughening additives are made by new processes. One process includes copolymerizing a (meth)acrylic ester with an olefin C2-20 by (i) admixing a (meth)acrylic ester, a greater than equimolar amount of a Lewis acid per mole of the (meth)acrylic ester, a free radical initiator and an olefin C2-20; (ii) and heating the resultant admixture at a temperature from about 60 to about 80xc2x0 C. for a time sufficient to permit copolymerization of the ester with the olefin. It may, under certain circumstances, be desirable to use a solvent to facilitate this reaction. Zinc chloride is an example of a desirable Lewis acid useful in the present invention. The molar ratio of olefin to (meth)acrylic ester is desirably from about 0.1 to about 10.
An additional process for preparation of copolymer toughening additives includes esterification of existing olefin/(meth)acrylate or olefin/alkenoic acid copolymers containing acidic functional groups to allow conversion of the undesirable, interfering acidic functionalities into esters, thus making the copolymers more suitable for use as additives for cyanoacrylate adhesives. The esterification process includes reacting the copolymer with an alcohol in the presence of a catalytically effective amount of an acid catalyst, at a temperature of about 50 to about 180xc2x0 C. and in the presence of a solvent which is capable of dissolving the copolymer and which is generally suitable for esterifications. The reaction is allowed to proceed for a time sufficient to allow conversion of the interfering carboxylic acid functionalities into esters. It is usually desirable to remove the acid catalyst after the reaction is complete by precipitating the copolymer in a solvent in which the catalyst is soluble and the copolymer is insoluble, e.g. methanol.
The present invention also provides cyanoacrylate adhesive compositions which are toughened by copolymers substantially free of acidic functional groups and acidic or basic impurities that can result in the over- or under-stabilization of the cyanoacrylate adhesive composition.
In another aspect of the invention there is provided a toughened cyanoacrylate adhesive composition which contains at least one cyanoacrylate monomer and a copolymer toughening additive soluble in the cyanoacrylate monomer, which copolymer toughening additive is substantially free of acidic functionalities and acidic or basic impurities and is the reaction product of an olefin C2-20 and (meth)acrylate ester.
In another aspect of the present invention there is provided a copolymer toughening additive which is at least partially, and desirably fully, soluble in a cyanoacrylate monomer and that, upon cure of the cyanoacrylate adhesive composition, undergoes a phase separation characteristic of an improvement in toughness. The present invention further provides a method for preparing the copolymer toughening additive which allows for copolymerization of alkenes (C2-20) with (meth)acrylate esters and provides a means to vary the degree of incorporation of the alkene into the copolymer additive. Moreover, the present invention seeks to provide a method of preparing a toughened adhesive composition that contains, as one of its components, the copolymer toughening additives herein described.
The present invention further relates to a process for preparing cyanoacrylate adhesive compositions which when cured exhibit increased toughness as compared to the same or similar adhesive compositions without the copolymer toughener additives. This process includes combining a cyanoacrylate monomer with a toughening additive which is the copolymer reaction product of an olefin C2-20 and a (meth)acrylate ester; subjecting the combined cyanoacrylate monomer and copolymer to conditions sufficient to allow the monomer to at least partially, and desirably fully, solvate the copolymer. On curing, the adhesive composition undergoes a phase separation of the copolymer additive from the polycyanoacrylate matrix.
A further aspect of the present invention includes providing a composition useful for toughening various types of adhesive compositions, and particularly cyanoacrylate adhesive compositions, which composition includes a copolymer toughening additive which is substantially free of acidic functionalities and acidic or basic impurities and which is the reaction product of an olefin C2-20 and a (meth)acrylate ester.
Finally, the present invention provides a process for sealing or adhering surfaces which includes the steps of applying the toughened adhesive composition of the present invention to a substrate surface, placing the surface in abutting relationship with another surface and permitting the adhesive composition to cure therebetween.